1. Field of the Invention
This invention relates to a magnetic memory device such as an MRAM, for example.
2. Description of the Related Art
As the conventional magnetic random access memory (which is hereinafter simply referred to as an MRAM), an example (U.S. Pat. No. 5,940,319, U.S. Pat. No. 5,956,267, European Patent Application Publication No. WO 00/10,172, Jpn. Pat. Appln. KOKAI Publication No. 8-306014) in which a yoke structure or a keeper layer formed of a magnetic material with high magnetic permeability is provided on the peripheral portion of a programming wiring of a memory cell is proposed.
The MRAM is a general term for solid memory devices in which storage information can be rewritten, held and read out as required by using the magnetization direction of a ferromagnetic body as a storage information carrier.
The MRAM memory cell generally has a structure having a plurality of ferromagnetic bodies laminated. Storage of information is performed based on binary information items “0”, “1” indicating that the relative magnetization (spin) directions of a plurality of ferromagnetic bodies configuring the memory cell are parallel or bilinear. Programming of storage information is performed by reversing the magnetization direction of the ferromagnetic body of each cell according to the current magnetic field caused by passing a current through programming lines arranged in a cross-stripe form. The power consumption of the MRAM at the storage information holding time is zero in principle and the MRAM memory cell is a nonvolatile memory device which can hold storage information even after the power supply is turned OFF. Reading-out of storage information is performed based on a phenomenon that the electrical resistance of the memory cell varies depending on the relative angle between the sense current and the magnetization direction of ferromagnetic body configuring the cell or the relative angle between the magnetization directions of a plurality of ferromagnetic layers, that is, on a so-called magneto-resistance effect.
It is understood that the MRAM has the following advantages by comparing the function of the MRAM with the function of the conventional semiconductor memory device using a dielectric substance. That is, the MRAM is a perfect nonvolatile memory and has various advantages that it can perform the rewriting operation by 1015 times or more, the readout cycle can be shortened since the nondestructive readout operation can be performed and the refresh operation is not necessary, and it is highly resistant to radiation in comparison with a charge coupled memory cell, for example. It is predicted that the integration density per unit area, programming time and readout time of the MRAM are approximately equal to those of a DRAM. Therefore, it is expected that MRAMs may be applied to external memory devices of mobile device, LSI hybrid devices and main memory devices of personal computers by utilizing the significant feature of non-volatility.
In the MRAM which is studied to be put into practice, an MTJ element exhibiting the ferromagnetic tunnel effect is used for the memory cell. The MTJ element is configured by a three-layered film which is mainly formed of a ferromagnetic body/insulating layer/ferromagnetic body and a current tunnels and flows through the insulating film. The tunnel resistance varies in proportion to the cosine of the relative angle between the magnetization directions of the two ferromagnetic metal layers and is set to a maximum value when the magnetization directions of the two ferromagnetic metal layers are bilinear. The state is set as “0”. On the other hand, when the magnetization directions of the two ferromagnetic metal layers are parallel, the tunnel resistance is set to a minimum value and the state is set as “1”.
However, the wiring current cannot be reduced when the conventional MTJ element is used and there occurs a problem that interference between magnetic fields occurs at the programming time.